Interaction between iron(II) and hydroxamic acids: oxidation of iron(II) to iron(III) by desferrioxamine B under anaerobic conditions

Interaction between iron(II) and acetohydroxamic acid (Aha), a-alaninehydroxamic acid (a-Alaha), b-alaninehydroxamic acid (b-Alaha), hexanedioic acid bis(3-hydroxycarbamoyl-methyl)amide (Dha) or desferrioxamine B (DFB) under anaerobic conditions was studied by pH-metric and UV–Visible spectrophotometric methods. The stability constants of complexes formed with Aha, a-Alaha, b-Alaha and Dha were calculated and turned out to be much lower than those of the corresponding iron(III) complexes. Stability constants of the iron(II)–hydroxamate complexes are compared with those of other divalent 3d-block metal ions and the Irving–Williams series of stabilities was found to be observed. Above pH 4, in the reactions between iron(II) and desferrioxamine B, the oxidation of the metal ion to iron(III) by the ligand was found. The overall reaction that resulted in the formation of the tris–hydroxamato complex 1 [Fe(HDFB)] and monoamide derivative of DFB at pH 6 is: 21 1 1 1 1 2Fe 13H DFB 52[Fe(HDFB)] 1H DFB–monoamide 1H O14H 4 3 2 Based on these results, the conclusion is that desferrioxamine B can uptake iron in iron(III) form under anaerobic conditions

[Tl(dota)](-): An Extraordinarily Robust Macrocyclic Complex.

The X-ray structure of {C(NH2)3}[Tl(dota)].H2O shows that the Tl3+ ion is deeply buried in the macrocyclic cavity of the dota4- ligand (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) with average Tl-N and Tl-O distances of 2.464 and 2.365 A, respectively. The metal ion is directly coordinated to the eight donor atoms of the ligand, which results in a twisted square antiprismatic (TSAP') coordination around Tl3+. A multinuclear 1H, 13C, and 205Tl NMR study combined with DFT calculations confirmed the TSAP' structure of the complex in aqueous solution, which exists as the Lambda(lambdalambdalambdalambda)/Delta(deltadeltadeltadelta) enantiomeric pair. 205Tl NMR spectroscopy allowed the protonation constant associated with the protonation of the complex according to [Tl(dota)]- + H+ left arrow over right arrow [Tl(Hdota)] to be determined, which turned out to be pKHTl(dota) = 1.4 +/- 0.1. [Tl(dota)]- does not react with Br-, even when using an excess of the anion, but it forms a weak mixed complex with cyanide, [Tl(dota)]- + CN- left arrow over right arrow [Tl(dota)(CN)]2-, with an equilibrium constant of Kmix = 6.0 +/- 0.8. The dissociation of the [Tl(dota)]- complex was determined by UV-vis spectrophotometry under acidic conditions using a large excess of Br-, and it was found to follow proton-assisted kinetics and to take place very slowly ( approximately 10 days), even in 1 M HClO4, with the estimated half-life of the process being in the 109 h range at neutral pH. The solution dynamics of [Tl(dota)]- were investigated using 13C NMR spectroscopy and DFT calculations. The 13C NMR spectra recorded at low temperature (272 K) point to C4 symmetry of the complex in solution, which averages to C4v as the temperature increases. This dynamic behavior was attributed to the Lambda(lambdalambdalambdalambda) Delta(deltadeltadeltadelta) enantiomerization process, which involves both the inversion of the macrocyclic unit and the rotation of the pendant arms. According to our calculations, the arm-rotation process limits the Lambda(lambdalambdalambdalambda) Delta(deltadeltadeltadelta) interconversion